fix musl build

Fix the following musl build failure:

receivedata.c: In function 'findSlice':
receivedata.c:348:2: error: unknown type name 'ptrdiff_t'
  348 |  ptrdiff_t pos = slice - clst->slices;
      |  ^~~~~~~~~
receivedata.c:17:1: note: 'ptrdiff_t' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?
   16 | #include "fec.h"
  +++ |+#include <stddef.h>
   17 | 

Fixes:
 - http://autobuild.buildroot.org/results/30208c6f175967fed6de690447a09e0c86547b24

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Upstream status:
https://udpcast.linux.lu/mailman3/hyperkitty/list/udpcast@udpcast.linux.lu/thread/4MBES2CPUSUNIZTC7NXCQYS2GAE4DQSG/]

diff '--color=auto' -Nura udpcast-20211207.orig/receivedata.c udpcast-20211207/receivedata.c
--- udpcast-20211207.orig/receivedata.c	2022-08-13 21:48:06.226588614 +0200
+++ udpcast-20211207/receivedata.c	2022-08-13 21:48:40.242992238 +0200
@@ -1,6 +1,7 @@
 #include <assert.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <sys/time.h>
 #include <errno.h>
